Microelectromechanical system (MEMS) devices typically include integrated circuitry or similar microelectronic elements along with micromechanical elements controlled by or otherwise communicated or interacted with the microelectronic elements. The micromechanical elements are generally formed utilizing photolithography, material deposition, etching and other fabrication processes which may be the same as or similar to those used for forming the associated integrated circuitry. Example MEMS devices include temperature sensors, pressure sensors, accelerometers, microswitches, micromirror spatial light modulators, and vibration responsive energy harvesters. Because the micromechanical elements are usually very sensitive and may be damaged or degraded by uncontrolled exposure to their operating environments, such devices typically require protective packaging. For applications requiring protection against moisture intrusion, hermetically sealed packaging is used to contain the micromechanical and associated microelectronic elements.
A MEMS device such as a Texas Instruments DLP® digital micromirror device (DMD), for example, has micromechanical elements that take the form of micromirrors located at pixel positions in arrays. The micromirrors are individually settable to “ON” or “OFF” orientations responsive to electric fields applied by microelectronic elements formed by CMOS processes at corresponding pixel positions in underlying memory cell arrays. To protect the mirrors, the micromirror arrays are located within cavities formed between the MEMS dies and light transparent covers. In order to benefit from economies of scale, at least some of the packaging steps are performed at wafer level, with a window wafer bonded to the die wafer prior to singulation.
In one packaging approach, an interposer wafer is inserted between the window wafer and the die wafer. The interposer is pre-formed with openings that define cavities to receive and surround the micromechanical components and acts as a spacing structure between the cover window and the MEMS device die. The window wafer is pre-manufactured to specified optical performance requirements relating to, for example, wavelengths to be transmitted, wavelengths to be blocked, aperture sizing, and anti-reflective (AR) coating material. For packaged MEMS devices like the DMDs, the interposer wafer is bonded to the MEMS wafer and the window wafer is bonded to the interposer wafer to form and maintain a seal while providing an optically suitable window for interaction between the micromirrors and the outside environment.
Formation of a cavity between interposer and substrate requires repeatable control of material removal processes, especially in a manufacturing environment. Strategies to use sourced material, such as pre-manufactured glass for lid substrate wafers, reduce cost while improving process manufacturability and repeatability.
Examples of prior approaches to fabricating packaged MEMS devices are given in U.S. Pat. Nos. 7,109,120; 7,118,234; 7,160,791; 7,787,170 and 7,833,879; the entireties of all of which are incorporated herein by reference.